Rate sedimentation of sheared chromatin from chick embryo fibroblasts (CEF) fractionated on glycerol gradients yields two chromatin fractions, a heavy (or fast sedimenting) component, and a light (or slowly sedimenting) component. cDNA prepared from avian myeloblastosis virus (AMV) hybridizes to equal extent with both the DNA moieties extracted from light and heavy chromatin fractions, indicating that the sequences hybridizing to AMV-cDNA are equally distributed between light and heavy fractions. Consequently, it is concluded that the endogenous proviral sequences are randomly integrated in the chromatin of CEF. However, when CEF were transformed by Rous sarcoma virus (RSV), which shares common sequences with AMV, molecular hybridization revealed that most of the exogenous proviral sequences were integrated within the light chromatin fraction. The results suggest that exogenous proviral sequences integrate in chromatin regions of particular configuration.